: The hypothalamic melanocortin-4 receptor (MC4-R) has been shown to have an important role in controlling energy balance in both rodents and humans. Neuronal fibers of at least two different types converge on sites of MC4-R expression in the paraventricular nucleus (PVH) of the hypothalamus. These include proopiomelanocortin (POMC) neurons that release both a-MSH and CART, and neurons that release both NPY and AGRP. Signaling via MC4-R appears to reflect a balance between a-MSH and AGRP release at the adipostat cells in the PVH that express this receptor. The detailed mechanism by which a-MSH and AGRP interact at MC4-R is unknown. It is also unclear how the product of the mahogany gene modulates the effect of AGRP on adipostat neurons. Two systems have been developed to allow further characterization of MC4-R signaling in the PVH. These include a hypothalamic slice preparation that permits electrophysiological recordings to be made from individual cells that are responsive (via GABAergic input) to both a-MSH and AGRP analogs. The second system involves a cultured hypothalamic neuronal cell line (GT-1) that expresses mRNA for MC4-R, mahogany, and leptin, and demonstrates specific binding of an AGRP analog. The Specific Aims of the proposal are i) to define the signaling modalities used by MC4-R and verify the GABAergic interneuron hypothesis, ii) to determine the mechanism by which AGRP opposes a-MSH action in the PVH, and iii) to determine the mechanism by which mahogany facilitates the antagonism of a-MSH signaling by AGRP.